岩体作为一种天然地质结构，内部常存有褶皱、节理、断层等结构面，而且寒区岩体工程受循环冻融作用易在这些弱面引发细观损伤扩展，并由于阶段性开挖、运输等重复性荷载导致岩体变形失稳，对寒区工程施工建设及运营维护产生严重隐患。针对在冻融及循环荷载作用下的岩石裂隙扩展演化及破坏特征问题，本文以白砂岩为研究对象，对饱和完整岩样和饱和预制双裂隙岩样开展冻融循环试验，而后进行单轴压缩及循环加卸载试验，分析其力学性质及加卸载能量演化与分配规律；并利用CT技术，获得不同冻融次数与不同加卸载应力状态下的三维裂隙结构图像，结合细观参数，分析砂岩冻融细观损伤演化规律，评估加卸载过程中裂隙扩展及破坏特征；分析冻融砂岩加卸载损伤演化机理。主要研究结论如下：

（1）揭示裂隙砂岩冻融损伤演化特征。冻融循环促进岩石裂隙连通主要发生在冻融前期，冻融后期主要表现为孔喉等效半径增大，孔喉结构分布范围越来越广，裂隙砂岩孔喉结构发育更为剧烈。冻融产生的孔/裂隙成为支配异质性的主要因素，峰值强度及弹性模量随孔隙率增大而降低，当孔隙率较小时，峰值强度随异质系数增大而降低，其余情况异质系数的影响较小。

（2）研究冻融砂岩加卸载力学性能及能量演化规律。随着冻融次数的增加，试件单轴加卸载峰值强度不断降低，同时变形不断增大，相同冻融次数下，裂隙试件单轴加卸载峰值应变小于完整试件。能量密度随加载比呈非线性增长，未冻融砂岩弹性能相对耗散能迅速增长，随着冻融循环次数增大，弹性能与耗散能的累积速率差值逐渐减小；应变储能指数与冻融次数呈线性负相关，且完整与裂隙砂岩敏感性相当。砂岩损伤能呈二次多项式函数增长，而不同冻融次数间差异较小。

（3）分析冻融砂岩加卸载裂隙扩展及破坏特征。完整砂岩各层面孔隙率增量较为均匀，裂隙砂岩在岩桥区域及预制裂隙尖端所在层易出现损伤集中区。破坏阶段完整砂岩分形维数介于2.19~2.44之间，裂隙砂岩介于2.35~2.40之间。未冻融完整砂岩加卸载后呈斜穿中部的剪切破坏，冻融后完整砂岩两侧发生拉剪混合破坏，形成X状共轭斜面剪切，破坏模式受冻融循环影响显著。裂隙砂岩的裂纹首先在预制裂隙之间拉伸，其次预制裂隙周围形成外翼裂纹，最后整体发生滑移破坏，冻融对裂隙岩石加卸载破坏模式影响甚微。随着应力加载等级增加，损伤逐渐加剧，相同加载比下高冻融次数砂岩损伤程度更大，不同冻融次数砂岩加载至最后一级时损伤变量D_m 均小于1，岩样破坏但未丧失承载能力。

As a natural geological structure, rock mass often has structural planes such as folds, joints, and faults. Moreover, rock mass engineering in cold regions is easy to cause meso-damage expansion on these weak surfaces due to cyclic freezing and thawing. Due to repetitive loads such as staged excavation and transportation, rock mass deformation and instability have serious hidden dangers for construction, operation and maintenance in cold regions. Therefore, aiming at the propagation evolution and failure characteristics of rock cracks under freeze-thaw and cyclic loading, this paper takes white sandstone as the research object, and carries out freeze-thaw cycle tests on saturated intact rock samples and prefabricated double-crack rock samples. Then, uniaxial compression and cyclic loading and unloading tests are carried out to analyze the mechanical properties and the evolution and distribution of loading and unloading energy; The three-dimensional fracture structure images under different freeze-thaw times and different loading and unloading stress states were obtained by CT technology. Combined with the microscopic parameters, the microscopic damage evolution law of sandstone freeze-thaw was analyzed, and the fracture propagation and failure characteristics during loading and unloading were evaluated. The damage evolution mechanism of freeze-thaw sandstone under loading and unloading is analyzed.The conclusions are as follows:

(1) The freeze-thaw damage evolution characteristics of fractured sandstone were revealed. The freeze-thaw cycle promotes the connection of rock fissures mainly in the early stage of freeze-thaw. In the later stage of freeze-thaw, the equivalent radius of pore throat increases, and the distribution range of pore throat structure is wider and wider. The pore throat structure of fractured sandstone is more intense. The pores / cracks generated by freezing and thawing become the main factor governing heterogeneity. The peak strength and elastic modulus decrease with the increase of porosity. When the porosity is small, the peak strength decreases with the increase of heterogeneity coefficient.

(2) The mechanical properties and energy evolution of freeze-thaw sandstone under loading and unloading were studied. With the increase of the number of freeze-thaw cycles, the peak strength of the uniaxial loading and unloading of the specimen decreases continuously, while the deformation increases continuously. Under the same number of freeze-thaw cycles, the peak strain of the uniaxial loading and unloading of the fractured specimen is smaller than that of the complete specimen. The energy density increases nonlinearly with the loading ratio. The elastic energy of no freeze-thaw sandstone increases rapidly relative to the dissipated energy. With the increase of freeze-thaw cycles, the difference between the cumulative rate of elastic energy and dissipated energy gradually decreases. The strain energy storage index is linearly negatively correlated with the number of freeze-thaw cycles, and the integrity is equivalent to the sensitivity of fractured sandstone. The damage energy of sandstone increases in a quadratic polynomial function, and the difference between different freeze-thaw times is small.

(3) The fracture propagation and failure characteristics of freeze-thaw sandstone under loading and unloading are analyzed. The porosity increment of each layer of intact sandstone is relatively uniform, and the fractured sandstone is prone to damage concentration in the rock bridge area and the layer where the prefabricated crack tip is located. The fractal dimension of intact sandstone is between 2.19 and 2.44, and that of fractured sandstone is between 2.35 and 2.40. After loading and unloading, the intact sandstone without freeze-thaw shows shear failure obliquely through the middle. After freeze-thaw, the tensile-shear mixed failure occurs on both sides of the intact sandstone, forming X-shaped conjugate inclined plane shear, and the failure mode is significantly affected by freeze-thaw cycles. The cracks of the fractured sandstone first stretch between the prefabricated cracks, then the outer wing cracks are formed around the prefabricated cracks, and finally the overall slip failure occurs. The freeze-thaw has little effect on the loading and unloading failure mode of the fractured rock. With the increase of stress loading level, the damage gradually intensifies. Under the same loading ratio, the damage degree of sandstone with high freeze-thaw times is greater. The damage variable D_m of sandstone with different freeze-thaw times is less than 1 when it is loaded to the last time, and the rock sample is destroyed but does not lose its bearing capacity.

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